Electrostatic dust separator

ABSTRACT

In an electrostatic dust separator having a ionizing electrode axially disposed within a Venturi, an annular nozzle produces a spray of an auxiliary substance in the gas stream issuing from the Venturi, around a polarizing electrode. The coarse particles of the spray are electrified by influence and they attract and retain the finer ionized dust particles carried by the gas. The spraying nozzle may be disposed within an auxiliary Venturi to increase the velocity of the gas and to improve impact effects between the dust particles and the sprayed particles. The ionizing electrode is preferably formed with fins to enhance the ionizing corona discharge, while the polarizing electrode is on the contrary smooth to avoid any noticeable discharge effect. The gas to be treated may be fed into the inlet of the main Venturi by an injector nozzle to produce a negative pressure whereby a portion of the gas issuing from the main Venturi may be recycled into same through an annular passage provided around the said main Venturi.

United States Patent Vicard 14 1 June 13, 1972 [54] ELECTROSTATIC DUSTSEPARATOR 2,852,239 9/1958 Vicard ..261/1 16 X cm 1:22:21 :1223z1z:;:;;;;;;; [22] Filed: Feb. 3 1970 FOREIGN PATENTS OR APPLICATIONS159,412 8/1940 Austria ..261/118 [211 App! 8295 4,824 1886 GreatBritain. ...261/l16 421,811 12/1934 Great Britain ..55/107 ForeignApplication Priority Data 940,930 11/1963 Great Britain ..261/DIG. 54

Feb. 13, 1969 France ..6903675 March 24, 1969 France ..6907777 jgj' jgigfg'g Pggjff [52] US. Cl ..55/l07, 55/117, 55/122, [57] ABSTRACT55/124, 55/127, 55/129, 55/130, 55/134, 55/138, 55/146, 55/152 55/154,55/226, 55/238, 55/257, In an electrostatic dust separator having aionizing electrode 55/259, 55/319, 55/338, 55/345, 55/418, 55/456,axially disposed within a Venturi, an annular nozzle produces 5 5 3261/1 16, 2 1 17 2 1 1 2 a spray of an auxiliary substance in the gasstream issuing from 54 317/3 the Venturi, around a polarizing electrode.The coarse parti- 51 1m. 01. ..B03c 3/36, B03c 3 41 cles of the p y areelectrified y influence and y attract [58] Field of Search... ..55 5,10, 11, 7, 107, 117, and retain the finer ionized dust Particles carriedy the 5 g 119, 20 2 124, 2 23 30 4 The spraying nozzle may be disposedwithin an auxiliary Ven- 135 136, 137, 133, 139, 146 150, 151 152 154,turi to increase the velocity of the gas and to improve impact DIG 38129, 226 238 257' 259 319 338 345, effects between the dust particlesand the sprayed particles. 418,456; 261/1316 54 64, 116, 117, 8; Theionizing electrode is preferably formed with fins to 317/3 enhance theionizing corona discharge, while the polarizing electrode is on thecontrary smooth to avoid any noticeable [56] References Cited dischargeeffect. The gas to be treated may be fed into the inlet of the mainVenturi by an injector nozzle to produce a UNITED STATES PATENTSnegative pressure whereby a portion of the gas issuing from the mainVenturi may be recycled into same through an annul,333,790 3/1920Bradley "SS/I 52 X passage provided around the i main Ventmi 1,579,4624/1926 Wintermute ..55/ X 2,357,355 9/1944 Penney ..55/l07 7 Claims, 7Drawing Figures PATENTEnJuu 1 3 1912 3, 668 835 sum 1 or 2 I) INYENT R.

:wuf 91, VIC.

BY g

ELECTROSTATIC DUST SEPARATOR The present invention relates toelectrostatic dust separators of the kind comprising an axial highvoltage electrode disposed in a Venturi unit so as to ionize the dustparticles in suspension in the gas which flows through the said unit,the said particles being thereafter electrostatically separated from thegas itself.

In accordance with the present invention an electrostatic dust separatorof the kind above referred to, comprises means disposed downstream ofthe Venturi unit for producing in the gas stream which issues from thesaid unit a spray of particles of an auxiliary substance, theseparticles being appropriately electrified so as to attract and to retainthe ionized dust particles in suspension in the gas stream. Theauxiliary substance, which may be a divided solid or preferably a liquidsuch as water, may be sprayed around a polarizing electrode disposed inthe gas stream.

The axial ionizing electrode is conveniently formed of a rod providedwith thin radially extending longitudinal fins between which the gasstream may circulate so as to dislodge any dust particle which couldhave settled on the said electrode, while the polarizing electrode ispreferably made of a mere smooth rod.

A portion of the gas issuing from the outlet of the Venturi unit ispreferably recycled into the inlet thereof in order that the ionizeddust particles entrained by the recycled gas may form nuclei adapted toattract and to retain the dust particles in suspension in the untreatedgas.

In the annexed drawings FIG. 1 is a general vertical section of anelectrostatic dust separator according to the invention.

FIG. 2 reproduces to an enlarged scale the upper portion of FIG. 1, butshowing the butterfly valve at its transverse position.

FIGS. 3 and 4 are cross-sections taken along lines III-III and IV-IVofFIG. 1.

FIG. 5 is a view similar to FIG. 1, but illustrating a modifiedembodiment.

FIG. 6 is a transverse section taken along line VI-VI of FIG. 5.

FIG. 7 is a vertical section showing the lower portion of anotherembodiment of the invention.

The electrostatic dust separator illustrated in FIGS. 1 to 4 comprises avertical cylindrical casing 1 having at its upper end a converginginjector nozzle 2. This injector nozzle opens in front of the converginginlet 3 of a conduit comprising a neck portion 4 followed by a divergingportion 5, the said conduit thus forming a Venturi unit which is axiallymaintained within the outer casing 1 by radial arms such as 6.

An insulating cross-member 7 is provided in the vicinity of the lowerend or outlet of Venturi unit 3-4-5. This crossmember carries twovertical axial electrodes 8 and 9. The first one extends upwardlythrough the Venturi unit and it terminates in the vicinity of the neckof the said unit, while the other one 9 extends downwardly well belowthe lower end thereof.

As shown in FIG. 3 the first electrode 8, or ionizing electrode, isformed of a rod provided with a relatively large number of longitudinalradial fins 10 of quite reduced thickness which therefore have arelatively sharp outer edge. These fins may for instance be mounted inlongitudinal slots milled in the periphery of the rod and in which theyare retained by soldering or otherwise. As to the second electrode 9, orpolarizing electrode, it is formed of a smooth rod of relatively largediameter, as illustrated in FIG. 4.

Below the lower end of the lower electrode 9 there is provided withinthe casing 1 an annular nozzle 11 carried by a tube 12 which extendsthrough the conical bottom 13 of the casing, its outer end 14 beingadapted to be connected with an appropriate supply. The nozzle 11 is soarranged as to produce a slightly diverging annular spray, concentric tothe lower electrode 9 without any intermediate screen and at arelatively small distance thereof. This spray strikes the casing wall,as indicated at 15. The spraying nozzle 11 is supplied with anappropriate liquid under pressure, as for instance water, it beinghowever noted that it would also be possible to spray solid particles insuspension in a gas such as air. The nozzle, the liquid or thepulverized solid arc electrically conducting and the nozzle is grounded.

The bottom 13 of casing 1 has a lower outlet 16 closed by an appropriateair-lock device, while a lateral exhaust 17 is provided somewhat abovethe said bottom.

In the embodiment illustrated in FIGS. 1 and 2,the upper nozzle 2 isequipped with a valve 18 of the butterfly type, pivoted about atransverse horizontal axis, this valve, of circular contour, being ofsuch diameter that at the horizontal position (see FIG. 2), it does notfully close the nozzle but leaves an annular gas passage 19 between itsperiphery and the inner side of this nozzle.

Both electrodes 8 and 9 are connected by means (not illustrated) with anappropriate high voltage source, also not shown.

In operation the dust-laden gas enters the apparatus through nozzle 2,valve 18 being fully open as illustrated in FIG. 1. This gas flowsdownwardly in the form of a jet through the converging portion 3, theneck 4 and the diverging portion 5. There is thus obtained an injectoreffect whereby gas is sucked from the annular space which surrounds theVenturi unit 3-4-5. The gaseous mass thus realized flows downwardly at ahigh velocity along the ionizing electrode 8, the water vapor containedby the gas condensing on the dust particles in the neck portion 4, in aper se known manner. Owing to the presence of the sharp outer edges ofthe fins 10, the high voltage applied to electrode 8 results in asparkless electric discharge of the type generally called corona effect,between this electrode and the surrounding walls of the Venturi, wherebythe suspended particles are strongly ionized.

A substantial portion of the gas issuing from the lower end of theVenturi unit 3-4-5 is recycled upwardly between the said unit and easing1, together with the suspended particles, due to the injector effect ofthe jet from nozzle 2. The gaseous mass which flows through the Venturiunit is thus, so to speak, inseminated with already ionized particleswhich play the role of nuclei on which the other particles tend toagglomerate. This progressive growdi of the suspended particlesfacilitates their separation.

The remainder of the gas issuing from the Venturi unit 3-4-5 flowsdownwardly towards outlet 17 around the polarizing electrode 9 andthrough the spray 15 of relatively coarse particles issuing from thespraying nozzle 11. Owing to the vicinity of this high voltagepolarizing electrode 9, which due to its smooth outer surface producesno corona effect, these coarse particles become electrified by influenceas they issue from nozzle 11, in such a manner that the gas flowsthrough a zone which contains coarse particles carrying an electriccharge of opposed polarity with respect to the ionization charge. Thesecoarse particles therefore attract and retain electrostatically thefiner ionized dust particles in suspension in the gas, which aretherefore entrained towards the bottom 13 of easing 1, wherefrom theymay be periodically evacuated through the air-lock or like valve device16. Of course some of these coarse particles may be entrained by the gasthrough the outlet 17, but owing to their dimensions they are easilyseparated by means of a conventional sleeve filter, centrifugalseparator or similar apparatus, together with the finer particles whichthey have retained.

Owing to the insemination of the incoming gas with already ionizedparticles entrained by the recycled gas, the ionization effects withinthe Venturi unit 3-4-5 are extremely rapid. The velocity of the gas flowthrough the latter may therefore be quite high. The production offiltered gas is thus important and moreover the dust particles cannotsettle on the inner walls of the Venturi, which eliminates the frequentcleaning operations generally required in conventional electrostaticseparators. Owing to the particular construction of the ionizingelectrode 8, the gas stream may freely circulate between the successiveradial fins where dust particles cannot therefore be retained. When theflow of the incoming dust-laden gas is so small that the velocity of thegas jet issuing from nozzle 2 becomes insufficient, it is possible toreduce the effective cross-sectional area of this nozzle by closingvalve 18 (or more accurately by bringing it to the horizontal position),which of course increases the velocity of the jet.

It may furthermore be noted that the recycled gas stream is submitted toa sudden change of direction at the outlet of the Venturi unit 3-4-5, asshown by the arrows in FIG. 1. This entails a quite noticeablecentrifugal acceleration which tends to separate the biggest particlesfrom this stream. In other words the zone immediately below the Venturiunit acts as a centrifugal separator for the particles in the recycledgas stream, those which have grown by collecting and retaining smallerparticles within the Venturi, being selectively projected downwardlytowards the spraying nozzle 1 1.

It is obvious that the butterfly valve 18 is of no use when the flow(quantity per unit of time) of dust-laden gas to be filtered issubstantially constant. The inlet portion or the neck of the Venturiunit could be provided with helicoidal vanes imparting to the gas streama rotational motion of high angular velocity in order to separate thebiggest particles. In such a case the fins should also be helicoidal soas not to hinder circulation of the gas between them also care wouldhave to be taken that the vanes do not short-circuit the axial electrode(the latter could be of reduced length or the vanes could be made of aninsulating material). The apparatus could comprise a lower settlingchamber of large volume for a better separation of the coarse particlesfrom the gas before the latter reaches the outlet 17. The spray ofelectrified particles below the Venturi unit could be realized in anyappropriate manner. The inner side of the walls of the Venturi unit 34-5could be provided with fins or blades in order to enhance ionization ofthe incoming dust particles. In some cases the recycling could bedispensed with.

In the modification illustrated in FIG. 5 the Venturi unit 3-4-5 issurrounded by an intermediate cylindrical casing 20 of substantiallylarger diameter (about twice the diameter of the neck portion 4 of theVenturi), this intermediate casing being spaced from the main outercasing 1 so as to leave an annular passage for the recycled gas stream.The upper and lower edges of this intermediate casing 20 are joined withthe upper and, respectively, the lower end of the Venturi so as to formtherewith a single member enclosing an inner closed annular space, thismember being supported by the radial arms 6. Radial partitions such as21 are disposed within the said closed space between the Venturi unit3-4-5 and the intermediate casing 20. These partitions, as well as theintermediate casin g, the Venturi unit itself and the supporting arms 6are made of a conducting material such as a metal. The closed space mayfurther be filled with a conducting substance such as water.

With such an arrangement the Venturi unit 345 is only connectedelectrically with the intermediate casing 20 which is itself groundedthrough arms 6.

Under such conditions when a high-voltage is applied to the upperelectrode 8, the Venturi unit 3-4-5 becomes electrified by influence,which means that electric charges of a polarity opposed to that ofelectrode 8 appear on its inner surface, while charges of the otherpolarity flow freely from the Venturi unit towards the intermediatecasing 20 through the radial partitions 21 (and also through theconducting substance which fills the hollow space between the Venturiand the intermediate casing when such a filling is provided). Owing tothe distance between the Venturi unit 3-4-5 and the intennediate casing,the electrostatic field which appears between the venturi and the upperelectrode 8 is much stronger. Furthermore the ionization current whichcirculates between the electrode.

and the Venturi may freely flow towards the intermediate casing 20without determining on the inner surface of the Venturi localizedelectric charges which could limit the ionization effects.

It is obvious that similar considerations would apply to the lower spray15, namely that in order to enhance electrostatic influence effects thespraying nozzle 11 should be grounded through a connection ofsubstantial length. But in this case the said connection is comprised ofthe oblique tube 12 which may be provided relatively longwithout-requiring any particular arrangement for this purpose.

In the embodiment of FIG. 7 an additional Venturi unit 22-23 is disposedbelow the outlet of the diverging portion 5 of the main Venturi unitwith which it is connected by a perforated intermediate zone 24. Thiszone may be formed of a perforated plate or of a grid, as shown. Thespraying nozzle 11 is axially disposed within the additional Venturiunit 22-23 and it is carefully stream-lined, as illustrated. Its annularspray is so directed as to flare upwardly and to pass through theperforated zone or grid 24. The lower end or outlet of the additionalVenturi unit is flanged outwardly as shown as 25, so as to close theannular space between the said Venturi unit and the outer casing, apartfrom some holes 26 of reduced crosssection.

In operation the gas stream issuing from the diverging portion 5 of themain Venturi unit is considerably accelerated in the converging inlet 22of the additional Venturi unit. Under such conditions the impact effectsbetween the dust particles or agglomerates of dust particles and thecoarse liquid particles from the spraying nozzle 11 are greatlyenhanced. The coarse particles collect between the additional Venturiand the outer casing together with the dust particles which they haveentrained, and the liquid flows through the holes 26 towards the bottomof casing 1, wherefrom it is evacuated through the air-lock device 16.

The angle of the spray with respect to the axis of the apparatus in thezone 24 is preferably below 45.

It is further to be noted that the recycled gas stream passes throughthe grid 24 as indicated by the arrows 27.

I claim:

1. An electrostatic dust separator comprising:

a Venturi unit having an inlet and an outlet for a dust-laden ahigh-voltage ionizing electrode disposed axially of said Venturi andterminating substantially at the outlet thereof, said ionizing electrodebeing in the form of a rod having thin longitudinally extending radialfins to ionize dust particles in the gas within said unit, and saidionizing electrode having a predetermined polarity;

a high-voltage polarizing electrode forming an extension of saidionizing electrode beyond the outlet of said Venturi unit, saidelectrode being in the form of a smooth rod, said polarizing electrodehaving the same polarity as said ionizing electrode;

spraying means to produce around said polarizing electrode a spray ofparticles of an auxiliary substance;

and means to collect said sprayed particles together with the dustparticles which they have attracted and retained.

2. In a dust separator as claimed in claim 1, means to recycle into theinlet of said Venturi unit a portion of the gas issuing from the outletthereof.

3. In a dust separator as claimed in claim 1:

an outer casing axially enclosing said Venturi unit while leaving anannular gas-recycling passage around same;

an axial injector nozzle in said outer casing in front of the inlet ofsaid Venturi unit to receive the gas to be treated and to inject sameinto said inlet together with a portion of the gas issuing from theoutlet of said Venturi unit upstream of the spraying means, said portionthus being recycled through said passage.

4. In a dust separator as claimed in claim 3, a butterfly valve disposedwithin said injector nozzle, the diameter of said valve being such thatat its position transverse to said injector nozzle it leaves an annulargas passage through said injector nozzle.

5. In a dust separator as claimed in claim 1, an intermediate casingaxially enclosing said Venturi unit while being spaced therefrom, saidintermediate casing having a first and a second end respectively joinedto the inlet and to the outlet of said Venturi unit so as to determinetherewith a closed annular space, and said Venturi unit and saidintermediate casing being made of an electrical conducting material;

means within said closed annular space to form a radially directedelectrical connection between said Venturi unit and said intermediatecasing;

an electrically conducting outer casing enclosing said intermediatecasing while leaving an annular gas passage between itself and saidintermediate casing;

means to form a radially directed connection between said intermediatecasing and said outer casing;

means to recycle into the inlet of said Venturi unit through saidannular gas passage a portion of the gas issuing from the outlet of saidVenturi unit;

and means to electrically connect said intermediate cas-ing with saidouter casing.

6. An electrostatic dust separator comprising:

a Venturi unit having an inlet at one end for the dust-laden gas to betreated, and an outlet at the other end through which said gas issues;

a high-voltage ionizing electrode axially disposed in said Venturi unitto ionize dust particles in the gas flowing therethrough;

spraying means adjacent the outlet to produce in the gas issuing fromsaid Venturi unit a spray of particles of an auxiliary substance;

electrifying means adjacent the outlet to electrify said sprayedparticles with such a polarity that they attract and retain ionized dustparticles in the gas issuing from said Venturi unit;

and means adjacent said outlet to collect said sprayed particlestogether with the dust particles which they have retained;

an auxiliary Venturi unit disposed downstream of the first named unit toreceive the gas issuing from the outlet of said first-named Venturiunit, said auxiliary Venturi unit having an inlet portion disposedadjacent the outlet of the first named Venturi unit, and having a neckportion and an outlet portion with said spraying means and saidelectrifying means being so disposed within said auxiliary Venturi unitthat the spray of particles of said auxiliary substance is produced inthe vicinity of the neck portion of said auxiliary Venturi unit in orderthat said particles of said auxiliary substance may act on the dustparticles in a zone wherein the gas which contains said dust particlesflows with a high velocity.

7. An electrostatic dust separator comprising:

an elongated outer casing having a first end and a second end;

an injector nozzle disposed in axial alignment with the axis of saidouter casing and at the first end of said outer casing to receive thedust-laden gas to be treated;

a main Venturi unit axially disposed within said outer casing whileleaving an annular gas-recycling passage between said main Venturi unitand said outer casing, said main Venturi unit having an inlet portion, aneck portion and an outlet portion, with said inlet portion disposed inspaced relationship to said injector nozzle, said inlet portion beingdisposed between said injector nozzle and said second end, said inletportion being disposed downstream of said injector nozzle to receive thegas issuing therefrom, and at a distance therefrom so as to formtherewith a gas-recycling injector device;

a high-voltage electrode axially disposed within said main Venturi unitto ionize dust particles in the gas flowing therethrough;

insulating means to support said ionizing electrode;

an auxiliary Venturi unit axially disposed within said outer casingdownstream of said main Venturi unit to receive the gas issuingtherefrom and leaving an annular space between said outer casing andsaid auxiliary Venturi unit,

said auxiliary Venturi unit havingan inlet portion, a neck portion andan outlet portion, with said inlet portion of said auxiliary Venturiunit being spaced from the outlet portion of said main Venturi unit soas to leave an intermediate annular zone;

means to substantially prevent gas flow through said annular space;

a high-voltage polarizing electrode extending axially of and adjacent tothe inlet portion of said auziliary Venturi unit;

insulating means to support said polarizing electrode;

an annular spraying nozzle axially disposed in the outlet portion ofsaid auxiliary Venturi unit to produce an annular diverging spray ofelectrically conducting liquid particles which surrounds said polarizingelectrode and passes through said intermediate annular zone so as tocollect within said annular space together with the ionized dustparticles which they have retained;

means to supply an electrically-conducting liquid to said annularnozzle;

means to remove collected liquid and dust particles from said annularspace;

and means for exhausting gas from the second end of said outer casingdownstream of the outlet portion of said auxiliary Venturi unit.

1. An electrostatic dust separator comprising: a Venturi unit having an inlet and an outlet for a dust-laden gas; a high-voltage ionizing electrode disposed axially of said Venturi and terminating substantially at the outlet thereof, said ionizing electrode being in the form of a rod having thin longitudinally extending radial fins to ionize dust particles in the gas within said unit, and said ionizing electrode having a predetermined polarity; a high-voltage polarizing electrode forming an extension of said ionizing electrode beyond the outlet of said Venturi unit, said electrode being in the form of a smooth rod, said polarizing electrode having the same polarity as said ionizing electrode; spraying means to produce around said polarizing electrode a spray of particles of an auxiliary substance; and means to collect said sprayed particles together with the dust particles which they have attracted and retained.
 2. In a dust separator as claimed in claim 1, means to recycle into the inlet of said Venturi unit a portion of the gas issuing from the outlet thereof.
 3. In a dust separator as claimed in claim 1: an outer casing axially enclosing said Venturi unit while leaving an annular gas-recycling passage around same; an axial injector nozzle in said outer casing in front of the inlet of said Venturi unit to receive the gas to be treated and to inject same into said inlet together with a portion of the gas issuing from the outlet of said Venturi unit upstream of the spraying means, said portion thus being recycled through said passage.
 4. In a dust separator as claimed in claim 3, a butterfly valve disposed within said injector nozzle, the diameter of said valve being such that at its position transverse to said injector nozzle it leaves an annular gas passage through said injector nozzle.
 5. In a dust separator as claimed in claim 1, an intermediate casing axially enclosing said Venturi unit while being spaced therefrom, said intermediate casing having a first and a second end respectively joined to the inlet and to the outlet of said Venturi unit so as to determine therewith a closed annular space, and said Venturi unit and said intermediate casing being made of an electrical conducting material; means within said closed annular space to form a radially directed electrical connection between said Venturi unit and said intermediate casing; an electrically conducting outer casing enclosing said intermediate casing while leaving an annular gas passage between itself and said intermediate casing; means to form a radially directed connection between said intermediate casing and said outer casing; means to recycle into the inlet of said Venturi unit through said annular gas passage a portion of the gas issuing from the outlet of said Venturi unit; and means to electrically connect said intermediate cas-ing with said outer casing.
 6. An electrostatic dust separator comprising: a Venturi unit having an inlet at one end for the dust-laden gas to be treated, and an outlet at the other end through which said gas issues; a high-voltage ionizing electrode axially dIsposed in said Venturi unit to ionize dust particles in the gas flowing therethrough; spraying means adjacent the outlet to produce in the gas issuing from said Venturi unit a spray of particles of an auxiliary substance; electrifying means adjacent the outlet to electrify said sprayed particles with such a polarity that they attract and retain ionized dust particles in the gas issuing from said Venturi unit; and means adjacent said outlet to collect said sprayed particles together with the dust particles which they have retained; an auxiliary Venturi unit disposed downstream of the first named unit to receive the gas issuing from the outlet of said first-named Venturi unit, said auxiliary Venturi unit having an inlet portion disposed adjacent the outlet of the first named Venturi unit, and having a neck portion and an outlet portion with said spraying means and said electrifying means being so disposed within said auxiliary Venturi unit that the spray of particles of said auxiliary substance is produced in the vicinity of the neck portion of said auxiliary Venturi unit in order that said particles of said auxiliary substance may act on the dust particles in a zone wherein the gas which contains said dust particles flows with a high velocity.
 7. An electrostatic dust separator comprising: an elongated outer casing having a first end and a second end; an injector nozzle disposed in axial alignment with the axis of said outer casing and at the first end of said outer casing to receive the dust-laden gas to be treated; a main Venturi unit axially disposed within said outer casing while leaving an annular gas-recycling passage between said main Venturi unit and said outer casing, said main Venturi unit having an inlet portion, a neck portion and an outlet portion, with said inlet portion disposed in spaced relationship to said injector nozzle, said inlet portion being disposed between said injector nozzle and said second end, said inlet portion being disposed downstream of said injector nozzle to receive the gas issuing therefrom, and at a distance therefrom so as to form therewith a gas-recycling injector device; a high-voltage electrode axially disposed within said main Venturi unit to ionize dust particles in the gas flowing therethrough; insulating means to support said ionizing electrode; an auxiliary Venturi unit axially disposed within said outer casing downstream of said main Venturi unit to receive the gas issuing therefrom and leaving an annular space between said outer casing and said auxiliary Venturi unit, said auxiliary Venturi unit having an inlet portion, a neck portion and an outlet portion, with said inlet portion of said auxiliary Venturi unit being spaced from the outlet portion of said main Venturi unit so as to leave an intermediate annular zone; means to substantially prevent gas flow through said annular space; a high-voltage polarizing electrode extending axially of and adjacent to the inlet portion of said auziliary Venturi unit; insulating means to support said polarizing electrode; an annular spraying nozzle axially disposed in the outlet portion of said auxiliary Venturi unit to produce an annular diverging spray of electrically conducting liquid particles which surrounds said polarizing electrode and passes through said intermediate annular zone so as to collect within said annular space together with the ionized dust particles which they have retained; means to supply an electrically-conducting liquid to said annular nozzle; means to remove collected liquid and dust particles from said annular space; and means for exhausting gas from the second end of said outer casing downstream of the outlet portion of said auxiliary Venturi unit. 